The proposed research will focus on elucidating the functional role of HdacS (Histone Deacetylase 3) in the regulation of mammalian gene transcription, in which we will ultimately delineate the requirement of HdacS in hematopoietic development. To date many members of the HDAC family have been identified and shown to promote transcriptional repression through the specific recruitment of co-factors to form distinct nuclear complexes, however little is known about how individual HDACs function. During normal developmental processes HDACs play key roles in the regulation of gene expression, in turn regulating many aspects of cellular responses. HDACs have also been implicated carcinogenesis and as such HDAC inhibitors are emerging as viable cancer treatment options. However, HDAC inhibitors such as TSA exhibit too high of toxicity for use in patients, while depsipeptide Phase II trials showed high cardiovascular toxicity. These studies exemplify the need to more fully understand the biological role that specific HDACs play in the regulation of gene transcription that will ultimately affect cellular function. We are particularly interested in clarifying the transcriptional role of HdacS and the implications that the loss of HdacS has in hematopoiesis. We will address the specific role of HdacS in hematopoiesis through the following speicific aims: Specific Aim 1: Define the function of HdacS in transcription in vivo. A. Identify genes regulated by HdacS as a means to identify substrate specificity and transcriptional targets involved in hematopoiesis. B. Identify miRNAs regulated by HdacS thereby elucidating a novel mechanism of miRNA regulation as well as mechanism of HdacS biological action. C. Determine the molecular mechanism by which HdacS regulates gene expression by analyzing the structural requirements for HdacS repression. Specific Aim 2: Elucidate the in vivo requirements for HdacS during hematopoiesis. A. GFP/Mx-Cre mouse model to knock-out HdacS and follow hematopoietic cell lineages. B. Lysozyme-Cre mouse model to define the requirement of HdacS in myeloid differentiation. The relevance of this research to the public is in the form of enhancing the uses of HDAC inhibitors in therapies for cancer. Our studies set the ground work for analysis of specific Hdac's in biological processes, thus enabling the design of better HDAC inhibitors with less side effects.